"Under normal, natural conditions the oceans are a buffer for temperature changes in the atmosphere," says study lead author Yair Rosenthal of Rutgers University in New Jersey. "But right now, we are completely out of equilibrium." (See "What is Global Warming?")

Because the ocean depths are thought to absorb about 90 percent of the excess heat seen from global warming, increased attention has turned to their heat-absorbing capabilities in recent years.

The effect is expected to add almost two feet (61 centimeters) of sea-level rise, on average, to coasts worldwide in this century, leaving aside the effects of melting glaciers.

Seafloor Secrets

In the study, Rosenthal and his colleagues analyzed tiny, shelled creatures called foraminifera that pile up on the ocean floor and have preserved a record of past seawater temperatures from the last 10,000 years. (See also "Global Warming.")

The ratio of magnesium to calcium in the shells of the tiny single-celled creatures indicates the ocean temperatures at their time of formation. So the team could look at changes in this ratio to see shifts in ocean temperatures over time.

The scientists looked at two seafloor records from Indonesian water at depths of 1,480 to 3,018 feet (455 to 920 meters). The sites were selected because they are seen as representative of places where Pacific Ocean waters have mixed for millennia.

The study improves on recent work on past ocean temperatures, chiefly because it is looking at multiple sites from multiple depths, says paleoclimate expert David Anderson of the National Oceanic and Atmospheric Administration's National Climatic Data Center, located in Boulder, Colorado.

"Any story that can tell us how the upper 1,000 meters [of the ocean] warms and cools as the Earth's energy balance changes is of interest," says Anderson.

Ocean Thermometer

Overall, the ancient shells tell a story of long-term cooling of ocean waters, a drop ranging from 1.5 to 2.2 degrees Celsius (2.7 to 3.8 degrees Fahrenheit) across the depth, starting from roughly 7,000 years ago until the Middle Ages.

The records also discern a bump in temperatures during a warm period around the year 1200, followed by a drop during the Little Ice Age that stretched from 1550 to 1850.

"What we see is that it takes decades for the ocean to absorb changes in surface temperatures," Rosenthal says. "Clearly there is warming going on now, and there is some possibility of [the ocean] serving as a bigger buffer for [warming].

"But we need to understand the ocean better before we can say that with confidence," he cautions.